(WIRED) -- Now that the Anthony Weiner Twitter meltdown has pretty much played out, I'm surprised that there hasn't been much discussion of the butterfly wing-flap that brought him down: Twitter's rules of engagement when it comes to "following."

The success of a social network is largely determined by its settings. In 2006, an engineer named Jack Dorsey had an idea for a way for people to share short updates on their lives with friends and family.

Working with a small team, Dorsey and his colleagues began developing and testing the product. This included determining the built-in boundaries of the service, a process which would determine the breadth and purpose of the entire project.

Twitter was a simple idea but settings had to be just right, like the proper temperature for a soufflé.

Should the rules be very restrictive, to preserve privacy and intimacy? (Too much restriction would make the service less useful.) Or should they be expansive, and invite a wide circle to share one's status reports? (Too broad a channel would mean a depersonalized cohort.)

The breakthrough that enabled Twitter to become the wildly successful service it is now came from a twist that was much more significant than even its founders knew: They made it possible to "follow" someone's messages without requiring permission.

Essentially you would take out a subscription to someone's Twitter stream. You would follow your best friend or your brother in the same way you would follow Barack Obama, DeSean Jackson or the New York Times.

This was a break from the traditional two-way agreement that ruled communications in previous social systems. This changed Twitter from an asynchronous instant messaging system into a hybrid of a social network and broadcast medium.

"The relationship model was something that we debated a lot," says Evan Williams, who headed the company (Odeo) that created Twitter. "In the first version, by default you were private." (This quote is from a conversation I had with Williams in 2009, when I was working on a story for Wired.)

But then Dorsey's team came up with the idea that you could follow someone without them following you.

"That was really important," says Williams. "From my perspective, I wanted something like a blog relationship model. What I thought was beautiful about blogs as opposed to e-mail or anything else is it's completely up to the recipient of the information whether or not they consume it."

But Twitter was much more intimate than blogs. Following someone on Twitter was not exactly like setting up a blog feed or subscribing to a magazine. You became part of a visible community.

The relationship model was something that we debated a lot.--Evan Williams

In order to make this happen, Twitter made public the list of people you followed, as well as the list of people who followed you. You would notice when your friends followed the same people you did. You could make connections with other people who followed the same tweeters you followed.

To further encourage the community aspect of Twitter, the founders determined that by default, all messages would be public. Weirdly, one of the questions that came up during this discussion was whether people would be creeped out when it came to flirting and other personal issues.

"This openness was the result of a lot of thought around the way we had started recognizing that people were communicating online," says Noah Glass, who was part of the original team. (I interviewed Glass for the aforementioned story.) "We were on MySpace, and I got into a lot of trouble from various girls posting things on that thing out in the open, and I started thinking about openness -- we all started thinking about openness.

"We come from a world where privacy is important. But we realized that not everyone shared this feeling about privacy in that same kind of way," Glass says. "People were having really intimate discussions out for everyone else to read. I realized that that the level of privacy we thought was important, was not necessarily important to a certain group of people for a certain type of communication. And so making any conversation open and followable was something based on those systems that were becoming popular."

This thinking influenced the settings when Twitter, because of user demand, implemented a "reply" feature. The replies, just like any other tweets, were public.

But what if you wanted to have a truly private conversation, as with SMS or e-mail, with someone on Twitter? This presented a problem to those with a huge following.

One way to do this would be to allow users to send a private message to any other user, just as they can with e-mail. But celebrities and others with large followings didn't want to open a private channel to just anyone.

So Twitter decided that for "direct" messages there should be some limits. Direct, private messages could only be sent to someone who followed you. The fact that you followed someone meant that you'd probably be happy to hear from him or her. To have a back-and-forth conversation, then, both parties would have to be following each other.

We realized that not everyone shared this feeling about privacy in that same kind of way.--Noah Glass

Otherwise, once you hit a few keystrokes to specify that something is a direct message, sending a private tweet is not all that different from sending a public tweet.

The service has never really figured a way to foolproof the process. It is a rare Twitter user -- even an experienced one -- who has not mistakenly sent something intended as a direct message out into the public Twittersphere. Happens all the time.

These settings helped make Twitter catnip for loquacious politicians like Anthony Weiner, who used it to establish his feisty personality with a nationwide community.

But it also proved his undoing when he misused Twitter for sordid sexual contact with women.

Twitter's regimen of rules tanked Anthony Weiner in several ways.

First of all, it provided a public record of the initial contacts that Weiner made with the women.

Here was the apparent pattern of Weiner's inappropriate communications: a woman would tweet an encouraging public message to the legislator -- which would be public, the only way she could communicate to him. (By and large, since the women were political supporters and not thinking of themselves as material for his sexual fantasies, this wasn't a problem for them.)

Then Weiner would respond to them -- but his responses were constrained by the knowledge that his replies were public.

Weiner needed a more private channel of communication for flirtations up to and including pictures of his package. Since the women followed him already, he could send them direct messages. But to receive their replies, he had to follow them in return. Only then could he engage in flirting or sexual repartee.

Weiner seemed not to realize the extent to which Twitter's rules still made him vulnerable.

The women were publicly listed among those accounts he followed. Since he only followed around 200 people, these new followers seemed out of place among the politicians, journalists, and celebrities on his list.

It was all too easy for a political foe to notice that Weiner was adding young women (and in at least one case, a porn star) to his followers soon after a public exchange.

And that is exactly what happened. A right-wing activist noticed Weiner's pattern and then harassed the young women the congressman followed.

But even then, Weiner did not curtail his behavior. He simply unfollowed those women and found new ones to flirt with. His Twitter use was a train wreck waiting to happen.

The train wreck occurred when Weiner confronted the confusing rail yard that Twitter's founders never truly fixed: the inadequately drawn distinction between a public message and a direct one.

When Weiner decided to send a young woman a picture of his crotch -- wearing gray boxers that barely contained his tumescence -- he had already taken the step of following her. But he made a common mistake between a direct private message and a public reply, and sent the picture out to the tens of thousands of people who followed him.

That was so crazily egregious that Weiner's initial lies that his account had been hacked seemed plausible.

But the evidence of his deeper misbehavior was already out in the open: the thumbnails of the young women he followed, publicly available on his Twitter account.

Journalists, political rivals, and right-wing muckrakers had no problem finding multiple women who had flirted and even received more graphic photos via "private" Twitter messages. When asked about what happened, the women talked, and Weiner's original "I was hacked" story fell apart.

Weiner was caught in the social net, undone by a bunch of conversations several years earlier between some San Francisco geeks trying to figure out the settings of a cool new product.

The details of web product design had led to the pants being pulled down on a promising political career.

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